12.mass stroage system

Chapter 12: Mass-StorageChapter 12: Mass-Storage
SystemsSystems

12.2 Silberschatz, Galvin and GagneOperating System Concepts
Chapter 12: Mass-Storage SystemsChapter 12: Mass-Storage Systems
Overview of Mass Storage Structure
Disk Structure
Disk Attachment
Disk Scheduling
Disk Management
Swap-Space Management
RAID Structure
Disk Attachment
Stable-Storage Implementation
Tertiary Storage Devices
Operating System Issues
Performance Issues

ObjectivesObjectives
Describe the physical structure of secondary and tertiary storage
devices and the resulting effects on the uses of the devices
Explain the performance characteristics of mass-storage devices
Discuss operating-system services provided for mass storage,
including RAID and HSM

Overview of Mass Storage StructureOverview of Mass Storage Structure
Magnetic disks provide bulk of secondary storage of modern computers
Drives rotate at 60 to 200 times per second
Transfer rate is rate at which data flow between drive and computer
Positioning time (random-access time) is time to move disk arm
to desired cylinder (seek time) and time for desired sector to rotate
under the disk head (rotational latency)
Head crash results from disk head making contact with the disk
surface
 That’s bad
Disks can be removable
Drive attached to computer via I/O bus
Busses vary, including EIDE, ATA, SATA, USB, Fibre Channel,
SCSI
Host controller in computer uses bus to talk to disk controller built
into drive or storage array

Moving-head Disk MachanismMoving-head Disk Machanism

Overview of Mass Storage StructureOverview of Mass Storage Structure
(Cont.)(Cont.)
Magnetic tape
Was early secondary-storage medium
Relatively permanent and holds large quantities of data
Access time slow
Random access ~1000 times slower than disk
Mainly used for backup, storage of infrequently-used data,
transfer medium between systems
Kept in spool and wound or rewound past read-write head
Once data under head, transfer rates comparable to disk
20-200GB typical storage
Common technologies are 4mm, 8mm, 19mm, LTO-2 and
SDLT

Disk StructureDisk Structure
Disk drives are addressed as large 1-dimensional arrays of logical
blocks, where the logical block is the smallest unit of transfer.
The 1-dimensional array of logical blocks is mapped into the
sectors of the disk sequentially.
Sector 0 is the first sector of the first track on the outermost
cylinder.
Mapping proceeds in order through that track, then the rest of
the tracks in that cylinder, and then through the rest of the
cylinders from outermost to innermost.

Disk AttachmentDisk Attachment
Host-attached storage accessed through I/O ports talking to I/O
busses
SCSI itself is a bus, up to 16 devices on one cable, SCSI initiator
requests operation and SCSI targets perform tasks
Each target can have up to 8 logical units (disks attached to
device controller
FC is high-speed serial architecture
Can be switched fabric with 24-bit address space – the basis of
storage area networks (SANs) in which many hosts attach
to many storage units
Can be arbitrated loop (FC-AL) of 126 devices

Network-Attached StorageNetwork-Attached Storage
Network-attached storage (NAS) is storage made available over a
network rather than over a local connection (such as a bus)
NFS and CIFS are common protocols
Implemented via remote procedure calls (RPCs) between host and
storage
New iSCSI protocol uses IP network to carry the SCSI protocol

Storage Area NetworkStorage Area Network
Common in large storage environments (and becoming more
common)
Multiple hosts attached to multiple storage arrays - flexible

Disk SchedulingDisk Scheduling
The operating system is responsible for using hardware efficiently
— for the disk drives, this means having a fast access time and
disk bandwidth.
Access time has two major components
Seek time is the time for the disk are to move the heads to the
cylinder containing the desired sector.
Rotational latency is the additional time waiting for the disk to
rotate the desired sector to the disk head.
Minimize seek time
Seek time ≈ seek distance
Disk bandwidth is the total number of bytes transferred, divided by
the total time between the first request for service and the
completion of the last transfer.

Disk Scheduling (Cont.)Disk Scheduling (Cont.)
Several algorithms exist to schedule the servicing of disk I/O
requests.
We illustrate them with a request queue (0-199).
98, 183, 37, 122, 14, 124, 65, 67
Head pointer 53

FCFSFCFS
Illustration shows total head movement of 640 cylinders.

SSTFSSTF
Selects the request with the minimum seek time from the current
head position.
SSTF scheduling is a form of SJF scheduling; may cause starvation
of some requests.

SSTF (Cont.)SSTF (Cont.)

SCANSCAN
The disk arm starts at one end of the disk, and moves toward the
other end, servicing requests until it gets to the other end of the
disk, where the head movement is reversed and servicing
continues.
Sometimes called the elevator algorithm.

SCAN (Cont.)SCAN (Cont.)

C-SCANC-SCAN
Provides a more uniform wait time than SCAN.
The head moves from one end of the disk to the other. servicing
requests as it goes. When it reaches the other end, however, it
immediately returns to the beginning of the disk, without servicing
any requests on the return trip.
Treats the cylinders as a circular list that wraps around from the
last cylinder to the first one.

C-SCAN (Cont.)C-SCAN (Cont.)

C-LOOKC-LOOK
Version of C-SCAN
Arm only goes as far as the last request in each direction, then
reverses direction immediately, without first going all the way to the
end of the disk.

C-LOOK (Cont.)C-LOOK (Cont.)

Selecting a Disk-SchedulingSelecting a Disk-Scheduling
AlgorithmAlgorithm
SSTF is common and has a natural appeal
SCAN and C-SCAN perform better for systems that place a heavy
load on the disk.
Performance depends on the number and types of requests.
Requests for disk service can be influenced by the file-allocation
method.
The disk-scheduling algorithm should be written as a separate
module of the operating system, allowing it to be replaced with a
different algorithm if necessary.
Either SSTF or LOOK is a reasonable choice for the default
algorithm.

Disk ManagementDisk Management
Low-level formatting, or physical formatting — Dividing a disk into
sectors that the disk controller can read and write.
To use a disk to hold files, the operating system still needs to
record its own data structures on the disk.
Partition the disk into one or more groups of cylinders.
Logical formatting or “making a file system”.
Boot block initializes system.
The bootstrap is stored in ROM.
Bootstrap loader program.
Methods such as sector sparing used to handle bad blocks.

Booting from a Disk in WindowsBooting from a Disk in Windows
20002000

Swap-Space ManagementSwap-Space Management
Swap-space — Virtual memory uses disk space as an extension of
main memory.
Swap-space can be carved out of the normal file system,or, more
commonly, it can be in a separate disk partition.
Swap-space management
4.3BSD allocates swap space when process starts; holds text
segment (the program) and data segment.
Kernel uses swap maps to track swap-space use.
Solaris 2 allocates swap space only when a page is forced out
of physical memory, not when the virtual memory page is first
created.

Data Structures for Swapping on LinuxData Structures for Swapping on Linux
SystemsSystems

RAID StructureRAID Structure
RAID – multiple disk drives provides reliability via redundancy.
RAID is arranged into six different levels.

RAID (cont)RAID (cont)
Several improvements in disk-use techniques involve the use of
multiple disks working cooperatively.
Disk striping uses a group of disks as one storage unit.
RAID schemes improve performance and improve the reliability of
the storage system by storing redundant data.
Mirroring or shadowing keeps duplicate of each disk.
Block interleaved parity uses much less redundancy.

RAID LevelsRAID Levels

RAID (0 + 1) and (1 + 0)RAID (0 + 1) and (1 + 0)

Stable-Storage ImplementationStable-Storage Implementation
Write-ahead log scheme requires stable storage.
To implement stable storage:
Replicate information on more than one nonvolatile storage
media with independent failure modes.
Update information in a controlled manner to ensure that we
can recover the stable data after any failure during data transfer
or recovery.

Tertiary Storage DevicesTertiary Storage Devices
Low cost is the defining characteristic of tertiary storage.
Generally, tertiary storage is built using removable media
Common examples of removable media are floppy disks and CD-
ROMs; other types are available.

Removable DisksRemovable Disks
Floppy disk — thin flexible disk coated with magnetic material, enclosed
in a protective plastic case.
Most floppies hold about 1 MB; similar technology is used for
removable disks that hold more than 1 GB.
Removable magnetic disks can be nearly as fast as hard disks, but
they are at a greater risk of damage from exposure.

Removable Disks (Cont.)Removable Disks (Cont.)
A magneto-optic disk records data on a rigid platter coated with
magnetic material.
Laser heat is used to amplify a large, weak magnetic field to
record a bit.
Laser light is also used to read data (Kerr effect).
The magneto-optic head flies much farther from the disk
surface than a magnetic disk head, and the magnetic material
is covered with a protective layer of plastic or glass; resistant to
head crashes.
Optical disks do not use magnetism; they employ special materials
that are altered by laser light.

WORM DisksWORM Disks
The data on read-write disks can be modified over and over.
WORM (“Write Once, Read Many Times”) disks can be written only
once.
Thin aluminum film sandwiched between two glass or plastic
platters.
To write a bit, the drive uses a laser light to burn a small hole
through the aluminum; information can be destroyed by not altered.
Very durable and reliable.
Read Only disks, such ad CD-ROM and DVD, com from the factory
with the data pre-recorded.

TapesTapes
Compared to a disk, a tape is less expensive and holds more data,
but random access is much slower.
Tape is an economical medium for purposes that do not require
fast random access, e.g., backup copies of disk data, holding huge
volumes of data.
Large tape installations typically use robotic tape changers that
move tapes between tape drives and storage slots in a tape library.
stacker – library that holds a few tapes
silo – library that holds thousands of tapes
A disk-resident file can be archived to tape for low cost storage; the
computer can stage it back into disk storage for active use.

Operating System IssuesOperating System Issues
Major OS jobs are to manage physical devices and to present a
virtual machine abstraction to applications
For hard disks, the OS provides two abstraction:
Raw device – an array of data blocks.
File system – the OS queues and schedules the interleaved
requests from several applications.

Application InterfaceApplication Interface
Most OSs handle removable disks almost exactly like fixed disks
— a new cartridge is formatted and an empty file system is
generated on the disk.
Tapes are presented as a raw storage medium, i.e., and application
does not not open a file on the tape, it opens the whole tape drive
as a raw device.
Usually the tape drive is reserved for the exclusive use of that
application.
Since the OS does not provide file system services, the application
must decide how to use the array of blocks.
Since every application makes up its own rules for how to organize
a tape, a tape full of data can generally only be used by the
program that created it.

Tape DrivesTape Drives
The basic operations for a tape drive differ from those of a disk
drive.
locate positions the tape to a specific logical block, not an entire
track (corresponds to seek).
The read position operation returns the logical block number
where the tape head is.
The space operation enables relative motion.
Tape drives are “append-only” devices; updating a block in the
middle of the tape also effectively erases everything beyond that
block.
An EOT mark is placed after a block that is written.

File NamingFile Naming
The issue of naming files on removable media is especially difficult
when we want to write data on a removable cartridge on one
computer, and then use the cartridge in another computer.
Contemporary OSs generally leave the name space problem
unsolved for removable media, and depend on applications and
users to figure out how to access and interpret the data.
Some kinds of removable media (e.g., CDs) are so well
standardized that all computers use them the same way.

Hierarchical Storage ManagementHierarchical Storage Management
(HSM)(HSM)
A hierarchical storage system extends the storage hierarchy
beyond primary memory and secondary storage to incorporate
tertiary storage — usually implemented as a jukebox of tapes or
removable disks.
Usually incorporate tertiary storage by extending the file system.
Small and frequently used files remain on disk.
Large, old, inactive files are archived to the jukebox.
HSM is usually found in supercomputing centers and other large
installations that have enormous volumes of data.

SpeedSpeed
Two aspects of speed in tertiary storage are bandwidth and
latency.
Bandwidth is measured in bytes per second.
Sustained bandwidth – average data rate during a large
transfer; # of bytes/transfer time.
Data rate when the data stream is actually flowing.
Effective bandwidth – average over the entire I/O time,
including seek or locate, and cartridge switching.
Drive’s overall data rate.

Speed (Cont.)Speed (Cont.)
Access latency – amount of time needed to locate data.
Access time for a disk – move the arm to the selected cylinder and
wait for the rotational latency; < 35 milliseconds.
Access on tape requires winding the tape reels until the selected
block reaches the tape head; tens or hundreds of seconds.
Generally say that random access within a tape cartridge is about
a thousand times slower than random access on disk.
The low cost of tertiary storage is a result of having many cheap
cartridges share a few expensive drives.
A removable library is best devoted to the storage of infrequently used
data, because the library can only satisfy a relatively small number of
I/O requests per hour.

ReliabilityReliability
A fixed disk drive is likely to be more reliable than a removable disk
or tape drive.
An optical cartridge is likely to be more reliable than a magnetic
disk or tape.
A head crash in a fixed hard disk generally destroys the data,
whereas the failure of a tape drive or optical disk drive often leaves
the data cartridge unharmed.

CostCost
Main memory is much more expensive than disk storage
The cost per megabyte of hard disk storage is competitive with
magnetic tape if only one tape is used per drive.
The cheapest tape drives and the cheapest disk drives have had
about the same storage capacity over the years.
Tertiary storage gives a cost savings only when the number of
cartridges is considerably larger than the number of drives.

Price per Megabyte of DRAM, From 1981 toPrice per Megabyte of DRAM, From 1981 to
20042004

Price per Megabyte of Magnetic Hard Disk, From 1981 toPrice per Megabyte of Magnetic Hard Disk, From 1981 to
20042004

Price per Megabyte of a Tape Drive, From 1984-Price per Megabyte of a Tape Drive, From 1984-
20002000

End of Chapter 12End of Chapter 12

12.mass stroage system

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